Lobsters are as mysterious as they are delicious. Nobody knows exactly how old they can get, though some experts estimate that an undisturbed shellfish can crawl around the ocean floor for upwards of a century. For fisherman, calculating a lobster's age has largely been a game of guesswork.

Before now, scientists deduced a lobster's age judging by size and other variables. But it's now known that lobsters and other crustaceans, such as crabs and shrimp, grow one ring per year in hidden-away internal spots.

Marine biologists could already calculate a fish's age by counting the growth rings in the bony part of its inner ear, a shark's from the rings in its vertebrae, and a clam's from the rings on its shell. Lobsters and crabs, however, didn't previously appear to have any such identifying parts, namely because they shed their calcified shells every year.

But researcher Raouf Kilada and his colleagues found that lobsters and crabs, in fact, have two different body parts that don't molt off and therefore accumulate rings: The eyestalk (the thin connection between the eyeball and the head) and the gastric mills (a part of the stomach equipped with "teeth" to help the lobster break down food). These parts can be looked at under a microscope to accurately calculate a lobster's age, which is important for any commercially fished species to "ensure sustainability," said Raouf Kilada.

And just in case you ever wondered what a lobster hatching eggs looks like, well, here it is: